A well drilling technique, particularly in highly deviated wells involves using a lightweight drilling fluid or mud. The drilling fluid weight is not heavy enough to prevent upward flow in the well due to formation pressure. A drilling head controls the pressure at the surface.
One type of prior art drilling head utilizes an elastomer to seal against drill pipe while rotating during drilling operations. The seal is an annular member mounted on bearings. It has a smaller inner diameter than the drill pipe, causing it to stretch and frictionally engage the drill pipe. The seal is exposed to abrasive drilling fluids and, thus, wears out quickly. Also, these seals are unable to withstand mud pressure more than a fairly low level.